The most commonly used gas separation membranes consist of a polymeric material which is spun in the form of a hollow fiber or cast as a flat film in such a way that it has an integral dense skin supported on an open cell porous mass. Such a membrane is known as an asymmetric membrane. Alternately, gas separation membranes are also made by forming a very thin film of a permselective polymer on a microporous substrate. Such a composite membrane is generally formed by using two different polymers. The polymer used for the microporous support, also known as the substrate, is mechanically strong and chemically and thermally resistive. Substrate selection is generally predicated by its ability to resist the solvents which are used to dissolve the coating polymer. Therefore, a substrate polymer is typically chosen which is not swollen or dissolved by the coating solution. In the majority of cases, however, the coating polymer is soluble only in those solvents which are solvents for the substrate as well, thereby limiting the ability to make composite membranes from a wide variety of polymers.
Typically, composite membranes have been made by coating a pre-formed support, such as a flat sheet or hollow fiber, with a solution of the coating prepolymer or polymer followed by the removal of the solvent.
Purl, U.S. Pat. No. 4,756,932, teaches a process for applying a highly permeable coating on a hollow fiber substrate to produce a composite hollow fiber membrane. The hollow fiber substrate is passed through a polymeric solution capable of forming a coating on the substrate. After formation of the coating layer on the substrate, a portion of the solvent is removed by evaporation, followed by leaching in a non-solvent. It is specifically taught that the solution of polymeric coating material should be one which does not adversely react with or affect the substrate.
Bikson, et al., U.S. Pat. No. 4,826,599, disclose a process for producing hollow fiber membranes by coating a porous hollow fiber substrate with a dilute solution of a membrane forming material, partially evaporating the solvent, followed by coagulation and recovering the membrane. The solvents used in the coating solution are all non-solvents for the substrate. Kafchinski, et al., U.S. Pat. No. 5,213,689, disclose a method of coating polyolefin microporous hollow fibers by wet-spinning, or alternately by dry jet-wet spinning, through a spinning jet in which a fluoropolymer-containing fluid is applied to the outer surface of the fibers as they pass through the jet. In this process, the liquids used to dissolve the fluoropolymer are non-solvents for the polyolefin substrate.
Williams, et al., U.S. Pat. No. 4,840,819, disclose composite membranes prepared by applying a thin coating of a permeable membrane material to a porous base material having a controlled amount of liquid incorporated therein. Solvent from the coating is subsequently removed by drying. The presence of the liquid in the porous support layer precludes any appreciable penetration of the membrane material into the pores of the porous support layer.
Sluma, et al, U.S. Pat. No. 5,242,636 discloses a process for making a multilayer capillary membrane by guiding a hollow capillary support membrane through the central bore of a spinneret having one or several concentric annular slits through each of which a solution of one of the polymers forming the separating layer is applied.
The above-described prior art processes clearly indicate that certain combinations of coating polymer and substrate are not possible based on their solution characteristics and compatibility issues, especially where the solvency of the coating solution coincides with or falls within the solubility domain of the substrate material.
In addition to the above processes, several coextrusion techniques have been developed. Ekiner, et al., U.S. Pat. No. 5,085,676, disclose a process for preparing multicomponent membranes by casting two or more solutions of polymer and partially removing solvent from the side of the cast polymer that is to form the separation layer of the membrane. The membrane is then quenched to freeze its structure and the remainder of the solvent is then removed. Kusuki, et al., U.S. Pat. No. 5,1 41,642, disclose an aromatic polyimide double layered hollow filamentary membrane produced by concurrently extruding first and second spinning dope solutions through inner and outer annular openings in a hollow filament-spinning nozzle to form a double layered hollow filamentary stream and subsequently coagulating said stream to form a double-layered membrane.
The above coextrusion processes require coextrusion of two or more components for which the coagulation process can't be independently controlled which may result in difficulties in obtaining the desired membrane properties.